Assessment and simulation of potential landslide caused by the rainfall intensity in Batu City during 2021

Rain is the main factor causing landslides; rainwater can enter the soil through rock gaps and then accumulate in the impermeable layer; the layer then becomes a slip plane that triggers the landslide. This study aims to determine the level of influence of rainfall on landslide events in Batu City in 2021. Batu City is one of the main tourist cities in Indonesia, so security and safety are the main factors that must be considered. The method used in this research is a descriptive analysis of rainfall and landslide data. The analysis process was approached with a literature review and simple mathematical calculations visualized on a 2D map using GIS and Geostudio to simulate groundwater infiltration and safety factors. The results showed that the number of landslide events increased with increasing rainfall. The rainfall for Batu City is between 1749.15-2238.73 mm, while most landslides occur in January and February. Landslides in Batu City in 2021 occurred 78 times, most of which occurred in Songgokerto Village (15 incidents). The intensity of rainfall greatly influences the occurrence of massive landslides; apart from changing the physical properties of the soil or rock, the mechanical properties also change rapidly. The wetting-drying process will make the properties of soil particles and rock aggregates more vulnerable or the durability index of the soil and rock layers will decrease due to continuous rainfall.


Introduction
Landslides are influenced by various predisposing factors such as geology, topography, and hydrology, which vary so that the causes of landslides in various regions are different [1].Generally, the main factor affecting landslides is rain; the number of landslides tends to increase during the rainy season.Landslides caused by rainfall are common worldwide, causing great harm to social and economic development [2].An increase in water content in the soil plays an important role in triggering landslides, thus causing an increase in the amount and extent of landslide damage in various parts of the world [3].
Rainwater can enter the soil through rock fissures and then accumulate in the impermeable layer, thereby reducing the effective stress and the soil's shear strength [4].Rainfall is the most common triggering factor for initiating and reactivating translational landslides.Rainfall conditions are quite high, causing water to seep into the fracture of the sliding plane, thus increasing the pore water 1314 (2024) 012017 IOP Publishing doi:10.1088/1755-1315/1314/1/012017 2 pressure and reducing the effective shear resistance from the sliding surface.Two types of rain trigger landslides in Indonesia, namely the type of heavy rain that lasts a short time and normal rain that lasts a long time.Heavy rain effectively triggers landslides on slopes with soil easily absorbing water, such as sandy clay and sand.Normal types of rain can last for several hours to several days and can trigger landslides on slopes where the soil has low permeability, such as clay.
Batu City is one of the largest tourist destinations in East Java, so security and safety are the main factors that the local government must consider.Based on the data obtained from the Regional Disaster Management Agency (BPBD) of Batu City, the incidence of landslides in Batu City is relatively high, and it is proven that in 2021 Batu City experienced 78 landslides [5].This research focuses on analyzing the effect of rainfall on landslide events in Batu City in 2021.2021 was chosen based on the number of landslide events in Batu City since 2021, and 10 years earlier, the most landslides occurred in that year.The results of this research can be a reference for the government and surrounding communities to anticipate and increase awareness of landslides when entering the rainy season.

Material and Method
The research locations cover all areas in Batu City, East Java.Geographically, Batu City has an area of 202.30 km2 with an altitude of 680-1200 meters above sea level.Batu City is surrounded by 3 mountains: Mount Arjuna with a height of 3339 m; Mount Welirang with a height of 3156 meters; and Mount Panderman with a height of 2010 m.The areas dominated by mountains will certainly produce slopes with varying degrees of steepness, so the potential for landslides becomes even greater.The topography of Batu City consists of two different characteristics; the north and west are undulating and hilly areas located on the slopes of Mount Arjuno-Welirang and Butak-Kawi-Panderman.While the eastern and southern regions are relatively flat areas.The wavy topography makes it quite easy for soil movement to occur, resulting in landslides.The flowchart in Figure 1 shows the research flow.The method used in this study was a literature review based on secondary data that we obtained from several government agencies in Batu City, Indonesia.The data includes landslide event data obtained from the BPBD Batu City and rainfall data obtained from the Meteorology, Climatology, and Geophysical Agency (BMKG) Karangploso Malang.Then, we recapitulate the data to obtain the results of the number of landslides in 2021 and the intensity of rainfall in Batu City that year.To support this research, supporting data are needed that influence the factors that cause landslides, such as the slope obtained from BPBD and rock type data obtained from geological maps and previous research.
The recapitulation results are then presented in the form of a 2D map with the help of GIS software, that modelling aims to make the data easier to read and analyze.The recapitulation results are then compared to obtain a relationship related to the effect of rainfall intensity on landslides in Batu City in 2021.As a material for analyzing water's effect on the soil's potential for landslides, we made a simulation using the Geostudio software.In addition, a literature review of scientific articles discussing landslide events in Batu City supports information at the analysis stage.

Results and Discussion
Batu City is surrounded by several mountains, causing the area to have varied topography and slope conditions.The tilt of the slope is the second factor after rain that triggers landslides.The condition of the increasingly steep slopes can trigger the intensity of landslides due to low soil stability.Pore water pressure in the soil is one of the parameters that influences the effective stress and shear strength on slopes [6].When rainwater enters the soil surface, it will trigger material movement on the ground surface.Materials in soil, soil mixture, and rock debris will move in the direction of the slope plane as the pore water pressure in the soil increases.The additional load on the material caused by water will suppress the soil material and boulders, thus triggering the movement of material on the slope surface.

Figure 2. Batu City Slope Map
The amount and circulation of groundwater strongly influence landslides on slopes dominated by clay [7].The types of clay and sedimentary rocks found on the soil's surface can easily be passed and stored in water; generally, water will stop at the igneous rock layer because water is quite difficult to seep into the igneous rock.When the layer is saturated with water, it will become a slip zone [4], so the rock above it will easily move along the slope and escape its initial state.Water accumulation can occur on slopes due to saturation and ground shaking may trigger landslides [8].Based on the slope level (Figure 2), Batu City has the lowest slope of 2.1% and the highest is more than 40%.
Generally, the geological conditions of mountainous areas will be dominated by igneous rocks as basic constituent rocks and on the surface, will be dominated by sedimentary rocks and clay.Based on the geological map of the Kediri Quadrangle, Batu City is included in three rock formations: Old Anjasmara Volcanics Formation (Qpat), Kawi-Butak Volcanics Formation (Qpkb), and Upper Quaternary Volcanics Formation (Qv (pandermen)).The three rock formations are dominated by clay, tuff, breccia, and lava [9].Clay, sediment, tuff, and breccia dominate the soil surface to a depth of 50 meters, while lava rocks form the bedrock [10].Based on BPBD Data for Batu City in 2021, Batu City experienced 78 landslides; this number is considered very big for the area of Batu City, which is 199.09km 2 .Most landslides occurred in Songgokerto Village (15 Incidents), Sumber Brantas Village (13 Incidents), and Gunungsari Village (12 Incidents) [5].Given the many landslides, some of these villages need special attention.The rainfall data we obtain is in the form of daily rainfall per region.We then model the data in the form of a rainfall distribution map; this is intended to make it easier to read the data.Based on Figure 3   with moderate rainfall.Theoretically, other factors have an effect, causing areas with moderate rainfall to trigger many landslides, such as the degree of slope and land use.Based on Figure 4 (a), the number of landslide events per month is linear, with an increase in monthly rainfall in Figure 4 (b), where the most landslide events occur in January and February 2021.Based on Figure 5, the 2020-2021 rainy season peak in East Java province occurs in January and February.As for November, the number of landslides has again increased along with increased rainfall.This event is the beginning of the rainy season in 2021-2022, which starts in November.The condition of clay soil and sediment that has not been passed by water for a long time will cause cracks on the soil surface; these fractures will then make it easier for water to enter the deeper parts of the soil, thus accelerating soil movement.The peak of the rainy season in Batu City occurs in January; this can be seen in Figure 5 that Batu City is symbolized by the number 66 (dark blue).The peak of the rainy season increased in rainfall intensity, thus triggering an increase in landslide events in Batu City; this is linear with the data shown in Figure 4.The peak of the rainy season causes water to enter the soil continuously and causes the soil to become saturated with water.Eventually, the shear strength of the soil becomes weak, thus triggering landslides.Rainwater stored in the soil for too long causes the soil to lose its ability to bond with each other.This is because the presence of water in the soil can reduce the shear strength of the soil, especially soils with relatively high porosity, such as clay and sediment.This statement is supported by information showing that the most landslides in Batu City in 2021 occurred during the peak of the rainy season compared to previous years.This condition is an important concern if rainfall exceeds 2238.73 mm; the potential for landslides is also predicted to increase.A study of the potential for landslides can be carried out based on calculating the rainfall intensity in an area.This research can be developed by making calculations and mathematical formulas to determine the probability of landslides based on rainfall intensity.Determining the probability of the potential for landslides can be a reference for local governments in implementing strategies, anticipation, and efforts to mitigate landslide disasters.However, the study needs to be supplemented with in-depth data and information on climate change.
Hydrology in Batu City is divided into 3 (three) types: surface water, groundwater, and springs.Surface water in Batu City includes the Brantas River and its tributaries, an alternative surface water source.Apart from being a surface water source, these rivers can also trigger flooding if an overflow occurs, especially in tributaries.The rainwater circulation model that flows and seeps into the body of the slope can be simulated using finite element model analysis with the Geostudio Software.Rainwater infiltration is simulated using seepage analysis to observe slope stability behaviour as expressed by the safety factor due to the wetting-drying process.The analysis was conducted by modelling runoff and ground flow during the rainy season.We did this modelling on one of the slopes in Songgokerto village with Geostudio software, where the village experienced the most landslides in 2021.The results of the analysis and typical modelling of existing slopes during the rainy season are shown in Figure 6.The seepage analysis in Figure 6 shows the potential for infiltration during the rainy season; the topsoil formed from crushed sedimentary rock reaches a degree of saturation ( Sr = 100%) due to the addition of water content to the layer below it (lower silty clay to sedimentary clay rock with tuff, breccia, and lava).The infiltration forces due to rainwater weaken the particle bonding ability, the pore spaces are completely filled with water, and the soil shear strength (c and f) becomes gradually reduced [12].Furthermore, normal groundwater circulation conditions were disrupted due to the steady state flow pattern changing to a transient flow pattern with varying speeds.Rainfall of more than 1749.15mm, which occurs continuously, will affect the movement of fluid particles to be very erratic due to the mixing and rotation of particles between layers of soil or rock.As a result, momentum is exchanged from the surface runoff due to rainfall to the normal groundwater flow on a large scale (turbulent flow).Conditions during the rainy season also trigger pore water pressure to be higher than natural conditions or based on normal groundwater depth.
The initial moisture content correlates with the thickness of the particle and aggregate layers around the soil particles or material resulting from the breakdown of the rock layer (diffusion layer) [13], which has the effect of weakening the suction layer between soil particles.This means that the initial increase in water content will weaken the suction power of the matrix between soil particles and rock material that is destroyed due to water infiltration for a long time.Furthermore, the diffusion layer will weaken the structure of the relationship between soil particles or micro-aggregates, or the rock durability index will decrease, thereby causing the micro-aggregates to be more susceptible to external forces which are forced in nature.In addition, increasing the initial water content will reduce the effective pore ratio, thereby narrowing the seepage space and weakening the intrusion effect and increasing excess pore water pressure.Based on these conditions, the minimum Safety Factor (SF) in rainy season conditions is in the range of 0.926 to 1.026 (Figure 7) from several simulation cases and existing conditions in the field.This is very different from the dry season conditions, where the minimum safety factor is 1.20.This fundamental difference is caused by the intensity of rainwater which significantly affects the rate of destruction of soil particles and/or rock aggregates due to the wetting process during the rainy season and the drying process during the dry season, which occurs significantly, especially in the first two cycles at the start of the rainy season [14].Based on simulation testing, the critical moisture content is close to 24 to 32%.This indicates that the addition of water content during the rainy season is less than the critical value of soil water content under normal circumstances.Consequently, the slope's safety number (SF) becomes less than 1.0.However, the turning point occurred during the dry season when the volume weight of particles and/or aggregates with water stabilized as the water content decreased.

Conclusion
The rainfall for Batu City is between 1749.15-2238.73mm, while most landslides occur in January and February, the peak of the rainy season in 2021.Landslides in Batu City in 2021 occurred 78 times, most of which occurred in Songgokerto Village (15 Incidents), Sumber Brantas Village (13 Incidents), and Gunungsari Village (12 Incidents).The most landslides occurred in January and February, with 58 incidents, which is the peak of the rainy season in 2021.Based on Figure 4 (b), the maximum precipitation is between January and February, the peak of the rainy season in 2021, and November, which is the beginning of the rainy season the following year.The results showed that the number of landslide events increased with increasing rainfall.In addition to the changing physical properties of the soil or rock, the mechanical properties also change rapidly.The wetting-drying process will make the properties of soil particles and rock aggregates more vulnerable or the durability index of the soil and rock layers will decrease due to continuous rainfall above 1749.15mm.Susceptibility to rainfall infiltration is triggered by changes in soil flow patterns and pore water pressures in the subsoil formed by layers of crushed sedimentary rock.This change will move the soil layer towards the slope simultaneously according to changes in physical and mechanical properties or shear strength that occur.Normal groundwater circulation conditions are disrupted due to the steadystate flow changing into a transient flow pattern with changing speeds and eventually becoming turbulent flow.Conditions during the rainy season also trigger the pore water pressure to be higher than natural conditions.Based on the depth of the normal groundwater table, the soil density decreases, resulting in a movement of the soil mass towards the slope.

Figure 3 .
Figure 3. (a).Map of landslide events in every village in Batu City in 2021, (b).Batu City Rainfall Intensity Map 2021 (b), the highest rainfall is shown in red, 2238.73 mm, and the lowest rainfall is shown in green, 1749.15mm, while moderate rainfall is shown in yellow.There is a relation between Figures3 (a) and 3 (b), where the number of landslide events per urban village is linear with high rain intensity, such as Sumber Brantas village with 13 incidents.In addition, Songgokerto village 15 times, and Gunungsari 12 times

Figure 4 .
Figure 4. (a).Graph of monthly landslide events in Batu City in 2021, (b).Graph of monthly rainfall for Batu City in 2021

Figure 5 .
Figure5.The peak of rainfall for East Java Province in 2021[11]

Figure 6 .
Figure 6.Rainwater infiltration model simulation on slope area using Geostudio software

Figure 7 .
Figure 7. Simulation of stability analysis model during the rainy season with Geostudio software